1. Field of the Invention
The present invention generally relates to a disconnection detecting device, and in particular to a disconnection detecting device for a battery including battery blocks each constituted by mutually-series-connected unit cells, the device being configured to detect a disconnection of an electrical wire connecting a connection point connecting, the device configured to detect disconnection of an electrical wire connected between adjacently-connected pair of higher-ordered battery block and lower-ordered battery block to a ground terminal of a voltage detection unit configured to detect a voltage between both ends of a unit cell belonging to the higher-ordered battery block and a power supply terminal of a voltage detection unit configured to detect a voltage between both ends of a unit cell belonging to a lower-ordered battery block.
2. Description of the Related Art
A hybrid electric vehicle (HEV), which is powered by a conventional internal combustion engine propulsion system and an electric propulsion system, has two batteries, i.e., a low-voltage battery in order of 12 volts for starting the conventional engine and a high-voltage battery for driving an electric motor. In order to obtain a high voltage, the high-voltage battery includes a plurality of unit cells series-connected with each other, the unit cell being a secondary cell such as a nickel-metal-hydride cell and a lithium cell.
The above-described high-voltage battery experiences variation in a voltage between both ends, i.e., a state of charge (SOC), of each unit cell when charging and discharging is repeated. In charging and discharging of a battery, in terms of durability and safety of the unit cells, it is necessary that the charging be stopped when the unit cell having the highest SOC (or voltage between both ends) reaches an upper-limit SOC (or an upper-limit voltage between both ends), and discharging be stopped when the unit cell having the lowest SOC (or voltage between both ends) reaches a lower-limit SOC (or a lower-limit voltage between both ends).
Accordingly, existence of variation in the SOCs depending on the unit cells practically results in decrease in available battery capacity. In a case of the HEV, this further causes insufficient assist/regeneration functionality for supplying complementary battery energy to the engine when the automobile climbs a slope or regenerating energy for the battery when the automobile goes downhill, which may result in decrease in practical vehicle's engine power and fuel consumption.
In this respect, in order to equalize the SOCs of the unit cells, it is necessary to detect a voltage between both ends of each of the unit cells.
In a conventional approach, the unit cells are grouped into the plurality of battery blocks and a voltage-detection integrated circuit IC (voltage detection unit) is provided on a per-block basis, and the voltage between both ends of the each of the unit cells constituting the battery block is detected by means of the dedicated voltage-detection integrated circuits. Also, in order to lower a breakdown voltage of devices constituting the voltage-detection integrated circuit IC, the voltage-detection integrated circuits IC are individually powered by their corresponding battery blocks.
Specifically, the voltage-detection integrated circuit IC employs a positive electric potential of a highest-ordered unit cell of its corresponding battery block as its source electric potential, and a negative electric potential of a lowest-ordered unit cell of the corresponding battery block as a ground electric potential.
According to the above-described configuration, a connection point of mutually-adjacent higher-ordered battery block and lower-ordered battery block is connected via an electrical wire to two terminals, i.e., to a ground terminal of the voltage-detection integrated circuit IC corresponding to a higher-ordered battery block and to a power supply terminal of the voltage-detection integrated circuit IC corresponding to a lower-ordered battery block.
When a disconnection occurs in the electrical wire, a voltage between both ends of the lowest-ordered unit cell of the higher-ordered battery block and the highest-ordered unit cell of the lower-ordered battery block cannot be detected. Also, even when the electrical wire is disconnected, an electric potential occurs between the ground terminal of the voltage-detection integrated circuit IC corresponding to a higher-ordered battery block and the power supply terminal of the voltage-detection integrated circuit IC corresponding to a lower-ordered battery block. The electric potential will be determined in dependence on balance between an impedance of the voltage-detection integrated circuit IC corresponding to a higher-ordered battery block and an impedance of the voltage-detection integrated circuit IC corresponding to a lower-ordered battery block. For example, if these two impedances are one and the same, then the electric potential will be intermediate between a positive electric potential of the highest-ordered unit cell of the higher-ordered battery block and a negative electric potential of the lowest-ordered unit cell of the lower-ordered battery block.
When this intermediate electric potential is resulted, it is not possible to successfully detect occurrence of the disconnection, for the voltages between both ends, detected by the voltage-detection integrated circuit IC, of the lowest-ordered unit cell of the higher-ordered battery block and the highest-ordered unit cell of the lower-ordered battery block are substantially the same as in a normal state. On the other hand, if an impedance of the voltage-detection integrated circuit IC corresponding to the higher-ordered battery block is small, then the potential shifts to the positive electric potential of the highest-ordered unit cell of the higher-ordered battery block, and if an impedance of the voltage-detection integrated circuit IC corresponding to the lower-ordered battery block is small, then the potential shifts to the negative electric potential of the lowest-ordered unit cell of the lower-ordered battery block. It is not possible to discern whether occurrence of such shifting has been caused by an abnormal state of the unit cell or by a disconnection.
In view of this, a cell voltage measuring device disclosed for example in the patent literature 1 is advocated as the above-described apparatus for disconnection detection (for example, see Japanese Patent No. 3300309). The known cell voltage measuring device has resistors each connected between the both ends of each battery block ends so that resistance values of the resistor connected between the both ends of an adjacent higher-ordered battery block and a lower-ordered battery block are different from each other.
Thus, when the voltage between both ends of the battery block is measured in a case of the disconnection, a voltage is measured which is obtained by dividing the voltage between the positive electric potential of the higher-ordered battery block and the negative electric potential of the lower-ordered battery block by the resistor connected to the higher-ordered battery block and the resistor connected to the lower-ordered battery block.
Due to this, when the voltage between both ends of the battery block is detected in the case of the disconnection, a battery block voltage is detected that is not detected in a normal state, and this detected battery block voltage is used to detect the occurrence of the disconnection.
The above-described cell voltage measuring device, for example in a flying capacitor technique, has a large resistance and the disconnection can be successfully detected if charging time is sufficiently large. However, when the charging time is short, the voltage cannot be correctly measured. As has been discussed in the foregoing, when the cell voltage measuring device has the voltage-detection integrated circuit IC powered by the unit cell, the connected resistance is so large that it is difficult to supply power when a disconnection occurs. Also, cost increases when the power is supplied from the low-voltage battery using the DC/DC converter. Further, such conventional device are effective in a case where variation in voltage among the battery blocks is small (with 10 percent maximum specified), but it will erroneously detect occurrence of disconnection due to the large variation in voltage among the battery blocks (for example, see Japanese Patent No. 3300309).